1. Field of the Invention
The disclosed embodiments of the present invention relate to power conversion, and more particularly, to a flyback power converter which operates in continuous conduction mode and is able to realize the zero voltage switching, and related control method thereof.
2. Description of the Prior Art
Due to simpler circuit architecture and higher power conversion efficiency, the flyback power converter has been applied to various circuit designs broadly. However, there is large ripple in the output voltage of the flyback power converter, which causes the flyback power converter to be limited to low output power applications. Please refer to FIG. 1 and FIG. 2 together. FIG. 1 is a diagram illustrating a conventional flyback power converter 100, and FIG. 2 is a timing diagram of signals in the flyback power converter 100 shown in FIG. 1. As shown in FIG. 1, an input voltage V_IN is converted to an output voltage V_OUT by a transformer 130. A control unit 150 controls the ON/OFF state of each of a main switch element 110 and an auxiliary switch element 120, a transformer 152 is arranged to maintain isolation between the main switch element 110 and the auxiliary switch element 120, and a secondary side current I_S is outputted from a diode 140 and a capacitor 142. The main switch element 110 includes a transistor 111 having a body diode 112, coupled between the drain and source of the transistor 111, and a stray capacitor 113. The auxiliary switch element 120, which acts as an active clamping circuit, includes a transistor 121 having a body diode 122 coupled between the drain and source of the transistor 121, and a stray capacitor 123. As shown in FIG. 2, when the main switch element 110 is turned off (i.e., the voltage V_P is at a low voltage level), the flyback power converter 100 absorbs leakage inductance power of the transformer 130, which is stored into a capacitor 125, to lower the voltage ripple of the voltage V_D by utilizing the auxiliary switch element 120 (controlled by a voltage V_A). Therefore, the flyback power converter 100 is able to realize the zero voltage switching (ZVS) in continuous conduction mode (CCS), which achieves the objective of enhancing the power conversion efficiency.
It should be noted that, as shown in FIG. 2, during the period the main switch element 110 is turned off, the current I_A flows through the transformer 130 twice back and forth, and the area A1 enclosed by the current I_A and the time axis is very large. In addition, the area A1 is even larger than the area A2, which is enclosed by the current I_M during the period the main switch element 110 is turned on, and therefore the unwanted power provided to the primary winding of the transformer 130 is quite high. As can be seen from the waveform of the voltage V_D, there is current flowing continuously through the primary-side circuit of the transformer 130 during the period the main switch element 110 is turned off. That is, power consumption occurs in the flyback power converter 100 due to the circulating current. Even though the ZVS is realized to enhance the conversion efficiency, the saved power may be cancelled out due to the generated circulating current. In addition, there is phase shift in the secondary-side current (i.e., the current I_S), which causes that the current I_S increases gradually during the period the main switch element 110 is turned off and then falls off abruptly when the auxiliary switch element 120 is turned off. The root-mean-square (RMS) value of the current I_S may increase, which accompanies the increase in the power loss of the circuit elements in the secondary-side synchronous rectifier circuit of the transformer 130.
In brief, the conventional flyback power converter 100 has the problems resulting from large circulating current flowing through the active clamping circuit and the phase shift in the secondary-side current. The large circulating current may cause a relatively high conduction loss, and the phase shift in the secondary-side current may cause the secondary-side switch element to generate a higher voltage spike. Therefore, the switching loss is higher and the overall conversion efficiency is poor, which fails to achieve the major objective of enhancing the power efficiency.
Thus, there is a need for an innovative power converter capable of realizing ZVS in CCM operation to solve the above-mentioned power loss problem.